During laser welding workpieces, a capillary (e.g., providing a region in which a workpiece to be machined is locally melted) may be formed in a processing zone. Due to continuously high energy input into the capillary by the laser beam, metal vapor emerges from the capillary or from a melt bath surrounding the capillary and rises in the form of small particles in a workpiece region surrounding the processing zone. A part of the laser beam focused onto the workpiece by a laser processing head is absorbed by the metal vapor particles, which are thereby heated and therefore emit thermal radiation due to their very high temperature. Accordingly, a metal vapor flame is formed.
A spatial configuration (e.g., an extent) of the metal vapor flame may vary greatly as a function of time and position during processing of the workpiece. In correspondence to this behavior of the metal vapor flame, the temperature and, concomitantly, the refractive index likewise vary greatly as a function of time and position, such that a thermal lens is formed, which causes laser radiation to deviate from a desired level with high temporal dynamics. Temporally and locally inhomogeneous energy input into the workpiece due to the thermal lens has a negative effect on the quality of workpiece processing. For example, in the case of laser welding, such effects can lead to weld splatters, to weld bead variations in the form of irregular bead curvatures, or generally to a degraded weld bead geometry.
The formation of a metal vapor flame, as well as propagation of weld gases, smoke, and the like, furthermore entails the problem that the rising metal vapor or the particles contained in the rising welding gases are increasingly deposited on focusing optics of the laser processing head and degrade the functionality thereof (e.g., by thermally induced focal spot displacement). This problem is particularly pronounced as a result of metal splatters that occur during the laser processing because the metal splatters are accelerated in an uncontrolled manner out of the melt bath in the direction of the focusing optics and burn into the focusing optics.
In order to obtain an improved process result (e.g., an improved bead quality), the interaction between the laser radiation and the metal vapor flame can be reduced. This is achieved by maintaining a region of the focused laser beam that lies below the processing optics substantially free of a mixture of welding gases and hot ambient air by providing at least one gas flow that is directed onto the focused laser beam and that passes through the focused laser beam. The gas flow is generated by one or more gas nozzles and is directed obliquely onto the processing zone or obliquely onto a region upstream of the processing zone on the workpiece. In order to protect the focusing optics from metal splatters, a transverse air flow (e.g., a cross-jet) is furthermore generated by a cross-jet nozzle that is positioned close to the focusing optics. The transverse air flow passes through the focused laser beam transversely to a beam axis of the focused laser beam and deflects the metal splatters before they reach the focusing optics.